Gemstone carat weight calculator
No scale to hand? Enter the stone's measurements, pick the gem and the shape, and get an estimated carat weight. Includes a size chart for round stones.
- Estimated weight
- 1.17 ct
- In grams
- 0.234 g
An estimate, usually within about 10% for a well-proportioned cut. Weigh the stone for any sale.
Round stone size chart: mm to carats
Estimated weight of a well-proportioned round stone with a depth of 60% of its diameter.
| Diameter (mm) | Diamond (ct) | Sapphire / ruby (ct) | Emerald / aquamarine (ct) | Amethyst (ct) |
|---|---|---|---|---|
| 3 | 0.10 | 0.11 | 0.08 | 0.07 |
| 4 | 0.23 | 0.27 | 0.18 | 0.18 |
| 5 | 0.46 | 0.52 | 0.35 | 0.34 |
| 6 | 0.79 | 0.90 | 0.61 | 0.60 |
| 6.5 | 1.01 | 1.14 | 0.78 | 0.76 |
| 7 | 1.26 | 1.43 | 0.97 | 0.95 |
| 8 | 1.87 | 2.13 | 1.45 | 1.41 |
| 9 | 2.67 | 3.03 | 2.06 | 2.01 |
| 10 | 3.66 | 4.16 | 2.83 | 2.76 |
| 12 | 6.32 | 7.19 | 4.89 | 4.76 |
How it works
Weight in carats is roughly length × width × depth × a shape factor × (the stone's specific gravity ÷ 3.52). The shape factors are the ones commonly used for diamonds, and the density adjustment scales them to other stones.
It is an estimate
Real weight depends on the cut's proportions, so expect an error of around 10% either way. A scale is always the right answer for a sale; use this for quick sizing, remote quotes and sanity checks.
Why density matters
A 6 mm round sapphire weighs more than a 6 mm round amethyst of the same cut, because corundum is denser than quartz.
How to use it
- Choose the gemstone. The calculator adjusts for its density.
- Choose the cut shape.
- Enter length, width and depth in millimetres (for a round stone, enter the diameter as both length and width).
- Read the estimated weight in carats.
Frequently asked questions
How many carats is a 6 mm round stone?
For a well-proportioned round diamond, a 6 mm stone is about 0.75 ct. A sapphire of the same size is heavier because sapphire is denser; see the chart on this page.
How accurate is this?
It is an estimate. Depth and cut proportions change the weight, and the shape factors are averages.
Can I use it for cabochons?
It is built for faceted stones. For cabochons, weigh the stone.